1. Field of the Invention
The invention is concerned with electrical devices which depend for their operation on ionic motion. Such motion may be macroscopic involving movement of ions as between affixed electrodes or may be localized. Device uses include capacitors, electrolytic cells, and bolometers.
2. Description of the Prior Art
An emerging field of interest involves rigid electrical devices which depend for their function on some degree of ionic motion. Motion may be macroscopic with ions moving between electrodes--a field of interest here involves solid electrolytic primary or secondary cells; motion may be extremely local (or macroscopic where blocking electrodes are used) with devices functioning on the basis of attendant dielectric constants. In the latter case, dielectric constant may be strongly dependent upon frequency, as well as temperature or magnitude of applied electric field so that such devices may be utilized, as well, for critical measurement of such parameters.
Effort to date has largely, but not exclusively, concerned crystalline materials, for example, sodium beta alumina and related compositions for ionic conductivity (see Journal of Applied Electrochemistry, Vol. 1, pp. 153 (1971)). Attention on high dielectric constant capacitive devices has been directed toward crystalline ferroelectric materials, such as, substituted barium titanates in which ionic motion is localized within single crystalline unit cells. (See "Multilayer Ceramic Capacitors--Materials and Manufacture" by Z. F. Capozzi, pub. Sell Rex Co., Nutley, N.J. (1975).)
As in so many areas, the limitations inherent in the use of crystalline materials has posed problems--i.e., anisotropy, as well as anomalous effects at crystallite interfaces or, alternatively, practical difficulty in obtaining large sections of near-perfect single crystal material. Where macroscopic ionic motion is desired, crystalline materials pose a special problem in that permitted motion is due to an unusual combination of properties which are highly structure and direction dependent. As a consequence, significant ionic conductivity in crystalline material is a rare phenomenon.
As in other areas of device investigation, workers have recognized that many of the shortcomings associated with crystalline materials might be avoided in amorphous materials. A fairly extensive survey of glassy compositions which have been considered for ionic motion properties is contained in Journal of Non-Crystalline Solids, Vol. 21 (1976) p. 343. One of the more promising material classes is based on Li.sub.4 SiO.sub.4 and includes both non-stoichiometric variations, as well as compositions modified by additions of titanium. See Vol. 3 Journal of Applied Electrochemistry, p. 327 (1973). To date, realized ionic conductivity in amorphous materials have been at least two orders of magnitude below that observed in the best crystalline materials as measured near room temperature. (Titanium modified Li.sub.4 SiO.sub.4, while attaining values of 10.sup.-3 to 10.sup.-4 ohm.sup.-1 cm.sup.-1 at 300 degrees C is typically at a level of only about 10.sup.-7 ohm.sup.-1 cm.sup.1 at room temperature which compares with reported values for sodium beta alumina at room temperature of the order of 10.sup.-2 ohm.sup.-1 cm.sup.-1 (see Journal of Chemical Physics, Vol. 54 (1971) p. 414) or for lithium beta alumina at room temperature of the order of 10.sup.-4 ohm.sup.-1 cm.sup.-1 see Journal of Materials Science, Vol. 12 (1977) p. 15.)
There does not appear to be an extensive amount of work directed to limited motion ionic phenomena, for example, in capacitors or other devices depending upon high or variable dielectric constant, except in the particular case of ferroelectric materials.
From the device standpoint, high capacitance per unit area has been achieved by procedures directed toward fabrication of extremely thin dielectric layers rather than by increasing the degree of ionic motion to produce materials which, themselves, have high dielectric constants. A good example of this approach is the anodized tantalum capacitor which has a dielectric constant of about 30 and which, in thin layers typically yields capacitance values as high as 0.1.mu.F per cm.sup.2. Substituted barium titanate polycrystalline samples evidencing dielectric constants as high as 5,000 are discussed in "Multilayer Ceramic Capacitor--Materials and Manufacture," supra. Sample thicknesses as small as 1 mil result in capacitances as high as 0.2.mu.F per cm.sup.2. Neither of these prior art structures is substantially improved by increasing temperature.